Only print out the error message if ldap_modify_ext_s actually returns an error, and not success.

(closes issue #12438)
Reported by: gservat
Patches:
      res_config_ldap.c-patch-code uploaded by gservat (license 466)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@114320 65c4cc65-6c06-0410-ace0-fbb531ad65f3